This invention relates to a process for the manufacture of magnetic recording media, especially to the surface treatment of magnetic recording tape. 2. Description of the Prior Art
When a magnetic recording tape of the conventional type generally constituted by a flexible base of non-magnetic material supporting a magnetic layer containing finely-divided magnetic particles dispersed in a flexible binder is to be used in video recording, it must be capable of recording information at high density. This depends on the coercive force (H.sub.c), the residual magnetic flux density (B.sub.r), the rectangular ratio (R.sub.s = B.sub.r /B.sub.m in which B.sub.m is the retentivity), and the smoothness of the surface of the magnetic layer to be able to record at high density.
When magnetic tape is driven relative to a magnetic head, the magnetic layer contacts the head very closely. However, there are many irregularities on the magnetic layer, so that the magnetic head contacts only the convex parts of the irregularities. As a result, such irregularities lead to attenuation of the output signal (so-called spacing loss) and to the generation of noise.
Surface treatment, such as calendering the magnetic layer containing the finely-divided magnetic particles dispersed in the flexible binder to improve a surface state and the density of magnetic particles in the manufacturing process is known. This calendering process is performed by passing the magnetic tape between a rotatory steel roll and an elastic roll, which generally are in contact with the magnetic layer and the support base, respectively, at the proper temperature and pressure. If there are irregularities on the surface of such rolls, they cause an unevenness to be transferred to the surface of the magnetic tape, and this reduces the benefit of the surface treatment. Therefore, the rolls which are used in such calendering process are required to have no irregularities of hardness and to have high durability and heat resistance. Irregularities of hardness cause a lack of uniformity of pressure, so that the rolls cannot be used in manufacturing at high temperature.
The steel roll presents no problem because it can easily be made smooth by polishing, and its surface has high hardness and durability. On the other hand, in prior art cotton rolls, woolen rolls which comprise cotton, absorbent cotton, pulp or wool and used as the elastic roll results in a gathering or fiber, so there is a limit to the smoothness of the surface of such rolls. If the irregularity of hardness on the surface of the elastic roll exists, such irregularity transfers to the surface of the magnetic tape and has a bad influence on the signal-to-noise ratio.
Moreover, it is required that the hardness of the elastic roll be high to increase the density of magnetic particles in the magnetic layer. This relation is as follows: ##EQU1## where: 2h.sub.o is the nip width, or width of the contact surface between the elastic roll and steel roll;
W is the applied pressure per unit length of the rolls; PA1 E is Young's modulus of the elastic material; and PA1 D.sub.1 and D.sub.2 are the diameters of the rolls.
Because Young's modulus of the elastic material is low, i.e., the hardness is low, the nip width 2h becomes large, which causes the pressure per unit area of the magnetic tape to be low. This causes the density of the particles not to increase.
Heretofore, it has been thought to be impossible to smooth out irregularities on the magnetic layer of the magnetic tape by a dry-calendering process. Thus, the addition of lubricants to the tape surface and/or to the calendering rolls was believed to be necessary to attain a successfully smoothed surface. One such technique for injecting a lubricant into the tape coating during calendering is described in U.S. Pat. No. 3,398,011. Contrary to the assumptions of the prior art, this invention is directed to a successful dry-calendering process for smoothing irregularities in the surface of a magnetic tape.